GPU along with CPU rendering performance
I have tested this by rendering BMW scene at 256*240 (with tile size 128*180). So:
 1. It is true that during the beginning of render, GPU renders 1 tile and CPU cores render 3 tiles according to MSI Afterburner, which shows 99% GPU usage.
 2. Near the end of render, GPU usage becomes 0% while CPU cores render remaining 3 tiles slowly.